Labs and Monitoring on a GLP-1

Common questions

Why is baseline lab work important?

You cannot interpret changes without a reference point. A ferritin that drops from 80 to 40 during treatment looks different than one that was already at 40 before treatment. A vitamin D at 50 that drops to 25 is actionable; a vitamin D that started at 25 is already a deficiency to address up front. Baseline labs make later decisions easier.

What labs should I ask for at baseline?

HbA1c and fasting glucose (glycemic baseline). Full lipid panel (total cholesterol, LDL, HDL, triglycerides). Liver enzymes ALT and AST (contraindication check and baseline). Vitamin D. Ferritin and full iron panel. Vitamin B12 and folate. TSH if you have any thyroid symptoms or history. Kidney function (creatinine, eGFR) if there is any concern.

How often should I repeat labs during treatment?

Most practitioners recommend 3-month intervals during active titration and weight loss. Once at stable dose and weight, 6–12 month intervals for most markers. Patients with specific concerns (diabetes, prior iron-deficiency anemia, post-cardiac event) may need tighter monitoring per their prescriber.

What does a drifting-low HbA1c mean on Wegovy?

If you are not diabetic and your HbA1c drifts into the 4s during Wegovy treatment, you are probably not eating enough. This is a signal to audit protein and caloric intake, not a positive outcome. Severe under-eating compounds muscle loss and fatigue. Tell your prescriber and dietitian.

What about cholesterol changes during treatment?

Total cholesterol and LDL often drop during GLP-1 treatment. Part of this is reduced intake (less saturated fat, less alcohol), part is improved metabolic profile with weight loss. The drop can reverse after discontinuation if prior eating patterns return. Do not read a treatment-era cholesterol drop as durable without maintained habits.

What if my iron or ferritin drops?

Ferritin below 40–50 ng/mL is associated with hair growth issues and fatigue. Iron intake on a GLP-1 is often reduced (less red meat, smaller portions overall). Fix: increase dietary iron (beans, lean red meat if tolerated, fortified cereals), consider a supplement with your prescriber's input, and recheck in 3 months.

Do I need to monitor liver enzymes?

Your prescriber will decide cadence. Liver enzymes (ALT and AST) were part of the baseline contraindication check and are typically monitored in the first 6–12 months of treatment. Most patients see stable or improving liver enzymes during weight loss because fatty liver reduces. Rising enzymes warrant investigation.

What about kidney function?

GLP-1 medications are not generally considered nephrotoxic, but dehydration — which is more common on a GLP-1 — can strain kidney function. Patients with pre-existing kidney concerns should monitor eGFR per their prescriber. Adequate hydration (2–3 L daily) is the most actionable protection.

What about vitamin D and B12?

Both commonly drift low on reduced intake. Vitamin D deficiency is widespread in the general population regardless of GLP-1 status. B12 deficiency is more common in patients on acid-reducing medications (PPIs, H2 blockers) which many GLP-1 patients use for reflux. Baseline + periodic retest + correct if low.

Should I check body composition?

If access allows, yes. A DEXA scan at baseline and at 6–12 months gives the clearest picture of fat vs lean mass change — far more useful than the scale alone on a GLP-1. Consumer bio-impedance devices are less accurate but useful for trend tracking if you use the same device consistently.

References

1. Sehgal NKR, Tronieri JS, Ungar L, Guntuku SC. Self-reported side effects of semaglutide and tirzepatide in online communities. Nature Health. 2026. Published online April 10, 2026. (DOI)
2. Practitioner case material: Eliana Witchell, MSc, RD, CDE. Clinical notes, 2023–2026. Anonymized.